It is now recognised that the transition of microbial species from a free-living, suspended or planktonic state to a component of an adherent community involves fundamental behavioural change. Both dental caries and periodontal diseases are essentially dependent on the polymicrobial plaque that develops with increasing complexity after the initial colonization of salivary components that coat the tooth surface. While there has been extensive analysis of the growth of dental plaque organisms as mixed communities in the planktonic state, to date there is little knowledge of interactions between adherent bacteria. The proposed studies will employ two powerful and complementary new technologies to monitor changes in gene expression occurring during initial colonisation and maturation of a model biofilm. Recognition of the factorial expansion of complexity in polymicrobial systems limits detailed analysis to two key microorganisms, Streptococcus gordonii as a major early coloniser of the acquired salivary pellicle and Streptococcus mutans which is strongly implicated as a major pathogen initiating dental caries. Both organisms can be genetically manipulated. The technique of In Biofilm Expression Technology (IBET) will enable comprehensive analysis of new gene expression patterns during biofilm formation on saliva-coated hydroxyapatite while Proteome analysis will facilitate recognition of altered protein profiles, particularly at the cell surface. Structural and time course analysis of altered gene expression patterns in situ will be enhanced by Confocal Laser Scanning Microscopy. Regulatory mechanisms relating to surface adhesins, the influence of extracellular sugar polymers, the response to unfavourable environments including acidic conditions and to the activity of key enzyme activities will be a focus. These parameters are highly pertinent and are the subject of study at the Institute. This program of research will provide essential data necessary for a therapeutic strategy aimed at the establishment and maintenance of a non-pathogenic dental plaque.